358 Green Chemistry, 2nd ed
safety features that make a Chernobyl-type of catastrophe impossible and can prevent
almost any type of accident that would result in release of significant radioactivity.
The most cited problem with nuclear energy is the waste disposal problem. As
of 2006, use of the U.S. nuclear waste repository at Yucca Mountain in Nevada has
been delayed because of concerns over its safety, largely of a political nature. In the
meantime, spent fuel rods from years of nuclear power generation are stored in pools at
power plant sites. This option is not as bad as it sounds because the activity of spent fuel
decreases most rapidly during the years immediately following removal from a reactor.
The only way that nuclear energy can become an acceptable long-time energy option is
for spent fuel to be reprocessed. This reclaims the uranium which can be enriched in the
fissionable uranium-235 isotope and put back into nuclear fuel rods. Fission products
can be separated and put into waste disposal sites, where after about 600 years they will
have only about the same activities as the original uranium ore from which they were
extracted. The longer-lived actinides including fissionable plutonium formed by non-
fission neutron capture by uranium nuclei can be separated and put back into nuclear fuel
where they will be “burned” during reactor operation.
All of the energy alternatives discussed above deal with the generation of electrical
energy. Electrification of railroads would enable electricity to be used for a significant
fraction of transportation demand. However, energy sources are needed to power
automobiles, buses, trucks, and ships that cannot be tethered to an electrical power grid.
Elemental hydrogen using fuel cells is often cited as “the fuel of the future.” However,
to date, no satisfactory way has been found to carry enough of this gas, which is a liquid
(and a hazardous one at that) only at very low temperatures under pressure for vehicles
that may have to run for relatively long times before refueling. Such vehicles will have
to be powered by liquid fuels, which are now petroleum based and in short supply and
which all release greenhouse gas carbon dioxide.
The best sustainable alternative for producing liquid fuels is to make them from
biomass. Such fuels are greenhouse-gas-neutral, that is, the carbon in the carbon dioxide
released by their combustion came originally from the atmosphere by photosynthesis
(as did the carbon in fossil fuels, but over a vastly longer time frame). Biomass is now
used for liquid fuels in the form of ethanol made from the fermentation of grain or sugar
from sugarcane and diesel fuel made by esterifying plant oils, particularly soybean oil.
But these sources require a high-value raw material that is in demand for food and are
economic only because of substantial government subsidies. Efforts to extract sugars for
fermentation to alcohol from wood and crop byproduct sources including stalks, leaves,
and straw have proven difficult and uneconomical.
The best alternative for preparing liquid fuels from biomass sources is thermochemical
gasification, which produces a synthesis gas consisting of a mixture of carbon monoxide,
CO, and elemental hydrogen, H 2. The proportion of H 2 can be increased by reacting CO
with steam (H 2 O). The CO and H 2 can be combined in various proportions to produce a
wide range of fuels including methane, gasoline, jet fuel, and diesel fuel. The technology
for accomplishing these objectives is well known and long-established; for example, it
was practiced on a large scale in Germany during World War II and more recently by the
Sasol, South Africa, installation.